1. Field of the Invention
The present invention relates to an angular rate sensor and more particularly to an angular rate sensor which includes a plurality of vibrating beams. The sensor includes electrical compensation for bias errors due to differences in the beam geometry, thus obviating the relatively labor-intensive mass balancing compensation method of the beams currently used in the art.
2. Description of the Prior Art
Angular rate sensors are generally known in the art. Examples of such sensors are discussed in detail in U.S. Pat. Nos. 4,510,802; 4,590,801; 4,654,663; 4,665,748; 4,799,385; and 5,396,797. Such sensors are used to provide a signal representative of the rate of change of angular motion relative to the longitudinal axis of the sensor for use in navigational and inertial guidance systems.
In a known implementation of an angular rate sensor, for example, as disclosed in the '663 patent, a plurality of vibrating beams are mounted to the structure whose angular rate is to be sensed by way of a gimballed mounting structure. The vibrating beams are typically made from a flat sheet or web of a crystalline material, often piezoelectric quartz. The beams are configured in a spaced-apart, parallel relationship and are excited to vibrate at their resonant frequency in the plane of the web, 180 degrees out of phase with one another. Angular motion about an axis parallel to the longitudinal axis of the beams causes a force generally perpendicular to the plane of the vibrating beams, known as a Coriolis force. The Coriolis force amplitude modulates the vibrating frequency of the beams, which, in turn, provides a measure of the Coriolis force and hence the angular rate of motion relative to the longitudinal axis of the sensor.
Unfortunately, small differences in the beam geometry can result in bias errors in the sensor output. Such bias errors are discussed in detail in the '385 patent, hereby incorporated by reference. More particularly, quadrature acceleration components resulting from acceleration of the beams during excitation are normally equal and thus cancel out since the beams are 180 degrees out of phase with one another. However, when the beams are not perfectly matched, the quadrature acceleration components of the beams may vary in magnitude and phase relative to one another and thus may not cancel out. The quadrature acceleration components are normally orders of magnitude larger than the angular rate signal. As such, the quadrature acceleration components can result in significant bias errors in the angular rate signal.
Typically, such bias errors are compensated by mass balancing of the beams. More particularly, gold is normally added to both vibrating beams and selectively removed by way of a laser until the biasing errors are either minimized or eliminated. Unfortunately, the process is relatively labor intensive and requires the balancing to be done by trial and error techniques.
One attempt to solve the problem of mass balancing of vibrating beams is disclosed in U.S. Pat. No. 5,113,698. In that system, a drive signal is provided to each beam to cause the beams to vibrate at a frequency that is relatively close to the isolated resonant frequencies of one of the beams in order to improve the mechanical Q of the sensor. However, the mechanical Q of an angular rate sensor is generally of little significance. More importantly for an angular rate sensor is that the quadrature acceleration components cancel.